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Rain shadow effects[edit]

Main article: Rain shadow

Washington experiences extensive variation in rainfall.
Rainfall in Washington varies dramatically going from east to west. The Olympic Peninsula's western side receives as much as 160 inches (4,100 mm) of precipitation annually, making it the wettest area of the 48 conterminous states and a temperate rainforest. Weeks may pass without a clear day. The western slopes of the Cascade Range receive some of the heaviest annual snowfall (in some places more than 200 inches or 5,100 millimeters water equivalent) in the country. In the rain shadow area east of the Cascades, the annual precipitation is only 6 inches (150 mm). Precipitation then increases again eastward toward the Rocky Mountains (about 120 miles (190 km) east of the Idaho border).
The Olympic mountains and Cascades compound this climatic pattern by causing orographic lift of the air masses blown inland from the Pacific Ocean, resulting in the windward side of the mountains receiving high levels of precipitation and the leeward side receiving low levels. This occurs most dramatically around the Olympic Mountains and the Cascade Range. In both cases the windward slopes facing southwest receive high precipitation and mild, cool temperatures. While the Puget Sound lowlands are known for clouds and rain in the winter, the western slopes of the Cascades receive larger amounts of precipitation, often falling as snow at higher elevations.^[89] Mount Baker, near the state's northern border, is one of the snowiest places in the world. In 1999, it set the world record for snowfall in a single season—1,140 inches (95 ft; 29 m).^[90]
East of the Cascades, a large region experiences strong rain shadow effects. Semi-arid conditions occur in much of Eastern Washington with the strongest rain shadow effects at the relatively low elevations of the central Columbia Plateau—especially the region just east of the Columbia River from about the Snake River to the Okanagan Highland. Thus, instead of rain forests, much of Eastern Washington is covered with dry grassland, shrub-steppe, and sand dunes.